Surface acoustic wave (SAW: Surface Acoustic Wave) filters and bulk acoustic wave (BAW: Bulk Acoustic Wave) filters are used as filters and duplexers for a high-frequency circuit used in wireless terminals such as mobile phone terminals. BAW filters have advantages of low-loss, steep skirt characteristic, high ESD (electro-static discharge) and high power durability as compared to SAW filters. The BAW filter is composed of a piezoelectric thin film resonator for example. Exemplary piezoelectric thin film resonators are a piezoelectric thin film resonator of an FBAR (Film Bulk Acoustic Resonator) type and a piezoelectric thin film resonator of an SMR (Solidly Mounted Resonator) type.
The piezoelectric thin film resonator of the FBAR type has a multilayered film formed by stacking a lower electrode, a piezoelectric film, and an upper electrode in this order on a substrate, and includes a space located below the lower electrode in a region (resonance region) in which the lower electrode and the upper electrode face each other across the piezoelectric film. On the other hand, the piezoelectric thin film resonator of the SMR type includes an acoustic reflection film formed by stacking a film having a high acoustic impedance and a film having a low acoustic impedance alternately with a film thickness of λ/4 (λ is a wavelength of an acoustic wave) instead of the space.
There has been known ladder-type filters including resonators connected to a series arm and a parallel arm in a ladder shape as a filter. The ladder-type filter have advantages of low loss, high suppression and high power durability, and thus are used in SAW filters and BAW filters.
In recent years, a large number of frequency bands are used due to the rapid widespread of mobile phones. This demands a steep skirt characteristic of a filter. For example, the filter is desired to have the steep skirt characteristic in a duplexer with a narrow guard band which is a frequency band between a transmission band and a reception band.
For example, an effective electromechanical coupling coefficient K2eff of a series resonator is made to be different from that of a parallel resonator in order to achieve the steep skirt characteristic at a high frequency side or low frequency side of a passband as disclosed in Japanese Patent Application Publication No. 2005-223808 (Patent Document 1). More specifically, the series resonator is a piezoelectric thin film resonator having the n-th harmonics as a principal vibration, and the parallel resonator is a piezoelectric thin film resonator having the m-th harmonics (m is equal to or more than (n+1)) as a principal vibration.
Japanese Patent Application Publication No. 2006-109472 discloses improving a Q-value of the piezoelectric thin film resonator by making thicker a thickness of a multilayered film in a resonance region in an outer peripheral portion. International Publication No. WO 2009/011148 discloses improving the Q-value of the piezoelectric thin film resonator and reducing a spurious by making thick the thickness of the multilayered film in an incline portion of a tip end of the lower electrode.
Patent Document 1 discloses using the piezoelectric thin film resonator having a fundamental wave as the principal vibration for the series resonator, and using the piezoelectric thin film resonator having the second harmonics as the principal vibration for the parallel resonator. The thickness of the multilayered film in the resonance region of the parallel resonator is made to be thicker than that of the series resonator in order to fabricate the above-described piezoelectric thin film resonator.
For example, it is desirable to control the K2eff with respect to each of piezoelectric thin film resonators composing the filter in order to achieve the steep skirt characteristic of the filter. However, the method disclosed in Patent Document 1 complicates a fabrication process because the thickness of the multilayered film in the resonance region needs to be changed in each piezoelectric thin film resonator. Furthermore, the K2eff is controlled by high-frequency vibration, and thus it is impossible to control the K2eff continuously due to non-contiguous values of the K2eff.